In a cellular radio communications system, mobile user equipment is generally free to move within the system and connect from one base station to another as it progresses between different serving cells. This cell crossover is generally denoted handover in the art.
In a Code Division Multiple Access (CDMA) communications system, a soft handover is offered to mobile user equipment, in which the mobile equipment at least temporarily is simultaneously connected to multiple cells in order to allow a smooth and seamless transition between cells. Thus, in this category of handover procedures, radio links are added and abandoned in such a manner that the user equipment always keeps at least one radio link to a (serving) cell in the system.
The handover function in a CDMA system is supported by user equipment-assisted signal quality measurements of a broadcast or beacon channel, e.g. Common Pilot Channel (CPICH), of its serving cell and any neighboring cells. The typical configuration is to setup such event-triggered quality measurements with measurement control messages from a serving Radio Network Controller (RNC) in the CDMA system. Such measurements could include the Received Signal Code Power (RSCP) of the CPICHs, ratio of energy level per received chip/bit to the noise spectral density (Ec/No, Eb/No) and/or path loss from the base station of respective cells, which is discussed in more detail in the 3GPP document [1]. In addition to information for which scrambling codes, i.e. the monitored neighboring cells, to report the signal quality measurements, the control messages from the RNC to the user equipment comprise a handover triggering threshold or margin. A handover procedure is then triggered based on a comparison of the measured signal qualities and the handover threshold.
This handover threshold is determined by the communication system and is communicated to the user equipment. The threshold determines the resulting size of a handover region between two cells. Conventionally, equal handover thresholds for all kinds of handover are employed through the system. The handover threshold will then be a compromise between two conflicting goals. Firstly, a large handover region is desired so that the user equipment travelling from one cell to another has time to measure, report, configure and synchronize on the new cell before the link to the old serving cell has to be dropped due to insufficient signal quality. Secondly, a relative small handover region is desired from a (downlink) radio resource point of view. Thus, a too large handover region will result in that the user equipment simultaneously is connected to multiple (downlink) radio channels during a rather long period of time. This means that the user equipment will, unnecessarily, occupy radio resources that could have been better used for other applications. Thus, the handover threshold has to be carefully chosen based on these conflicting goals and in some instances a fixed handover threshold for all kinds of handover procedures will result in reduced service of quality and possibly dropped calls for the user equipment.
In a US Patent [2], Hakalin and Hulkkonen disclose a method of dividing traffic between cells managed by a respective base station in a cellular radio system. The base station of a serving cell receives, from its connected mobile stations, power measurements of broadcast control channels from neighboring cells. The number of common available frequencies of the cell and the reported neighboring cells is calculated. Cells having at least one common frequency with the serving cell are tabulated in a table indicating, for each neighboring cell: its identifier, the ratio of the common frequencies of the serving cell and the neighboring cell and a ratio between the number of reported poor power quality measurements relative to all reported power measurements. The poor quality percentage in this table is then used for determining a connection ratio for each interfered neighboring cell. This connection ratio is, in turn, used for determining a correction factor that is used for dynamically modulating the handover threshold for the serving cell and its neighboring cells. Thus, a handover threshold that is based on the amount of traffic and interference in the system is obtained.
In a US Patent [3], Chandra et al disclose a method that generally determines optimal handover thresholds based on a computation of a power budget for each serving cell/neighbor cell combination in a cellular communications system and on a probability distribution function for each such cell combination. The determination of handover threshold will be a compromise between call quality and an evenly distributed traffic channel density among the cells in the system. The optimization is formulated as a nonlinear optimization of two objective functions subjected to nonlinear constraints that characterize the constraints on overlapping regions between neighboring cells, each cell's resource, the traffic generated within each cell, ping-pong margin between mutual neighbors and minimum/maximum allowable handover thresholds for acceptable call quality.
A major disadvantage of the above-identified two prior art solutions is that extensive data processing and calculations are required in order to determine the handover threshold for the serving cell/neighbor cell combinations.
Johansson et al discloses a solution for a soft handover procedure in an international patent application [4]. This handover procedure is essentially divided into two sub-procedures: a first preliminary portion of the handover sequence and another remaining portion. Time-critical handover activities such as receiver establishment for listening mobile user equipment and Layer 1 (L1, physical layer) uplink synchronization for the user equipment are performed during the preliminary handover sub-procedure whereas the remaining handover activities are initiated and finished during the second sub-procedure. The second (conventional) sub-procedure is associated with a fixed handover-triggering threshold. However, a dynamic threshold is employed for the preliminary portion of the handover sequence. This dynamic threshold depends on a probability that the user equipment will engage in soft handover and on a probability weight.
In order to determine the dynamic threshold of the document [4], statistical information of handover history of other mobile user equipment is required. Thus, a lot of prior user equipment measurements have to be stored and processed in order to calculate a present dynamic threshold. In addition, there is a compromise between two conflicting goals in determining the probability weight needed for calculating the dynamic threshold. Firstly, a large weight is required to make certain that the preliminary portion of the handover sequence is not started too late. However, a small weight is desired so that handover will not be started too soon and then waste hardware resources.